For various reasons, a PC I use has Java 1.4 installed and the latest Eclipse Europa requires 1.5. I am unable to update the classpath, environment etc. so although 1.5 was installed, Eclipse would not find it.

The problem was "More than likely the cause is that Windows renamed the drive to a letter that is already in use. This will happen if you have several card readers, thumb drives or external hard drives attached. It will also happen if you are on a network and have mapped drives".